Assembly of an integrated circuit (IC) for electronic package involves attachment of a die or chip to a substrate, such as a Bismaleimide-Triazine (BT) substrate or a silicon wafer, either with the chip facing up or down.
Wire bonding is a face-up bonding technique that uses electrically conducting wires to connect from Input/Output (I/Os) or electrodes of the chip to corresponding bond pads on the substrate or a bottom chip in an electronic package. The attachment of face-up chip is done by gluing the chip onto the substrate or the bottom chip by epoxy, and the connecting wires may be thin gold, copper, or aluminum wires.
Flip-chip bonding process is a face-down bonding technique where interconnections between electrodes of the chip, such as a flip-chip IC, and the bond pads of the substrate or the bottom chip are realized using bumps, such as balls, micro-bumps, or solder capped copper pillars, of electrically conducting material. The bumps are deposited over the electrodes of the flip-chip IC, which may be located at a periphery of the flip-chip IC, and were originally used for wire bonding interconnects. If the pitch of the electrodes is small, a redistribution layer may be needed to redistribute the electrodes from the periphery of the flip-chip IC to an area array across the surface of the flip-chip IC. In some high density I/O flip-chip IC, thousands of electrodes may be arranged in a combination of peripheral and array area or full area configuration.
Controlled collapse chip connection (C4) flip-chip process utilizes an oven to reflow the bumps on a chip which is accurately placed onto the substrate, thus realizing the flip-chip interconnections bonding process. If there are a large number of I/Os and the pitch is small, solder capped copper pillars are used, instead of micro-bumps which may be more susceptible to bridging failure during reflow. The reduction of the solder volume in the solder cap of the copper pillars may minimize the chances of solder bridging during solder reflow for fine pitch and high density I/O flip-chip IC. For example, the solder capped copper pillars may have diameters as small as 30 μm, and heights as small as 40 μm (comprising of 25 μm high copper pillar and 15 μm high solder cap). Nevertheless, after the flip-chip bonding process, it is preferred that the solder capped copper pillars have a high enough stand-off height for a subsequent underfilling process.
However, as the amount of solder in the solder capped copper pillars is reduced, some problems may be encountered when utilizing the oven reflow process using an ordinary reflow oven. For example, the advantage of self-alignment of solder joints in a solder reflow process would no longer exist, when there is too small an amount of solder in the solder capped copper pillar. A more accurate flip-chip bonding machine, for example having placement accuracy better than 2 μm to 3 μm, may be required to place the chip onto the substrate. Other solder joints related failures such as open and cold joints may be present due to the following reasons: (i) there may be uncontrollable die tilt since there is no compression force acting on the chip during the reflow process in the reflow oven; (ii) the chip may shift out of position due to vibration of a convey belt in the reflow oven; (iii) there may be height variation (for example up to 2 μm) between the different bond pads of substrate; and (iv) there may be excessive substrate warpage when the substrate is heated up to a high temperature in the reflow oven. In addition, poorer bonding material quality may also reduce the yield of the oven reflow process. It may be due to the fact that: (i) there may be height variation of up to 2 μm to 3 μm between the different solder capped copper pillars on the chip, and (ii) there may be volume inconsistency between the different solder caps on the solder capped copper pillars of the chip.
In some flip-chip bonding processes for thin dice, such as memory packages, a thin silicon chip, for example of thickness less than 50 μm, may exhibit substantial warpage even when it is free standing, which is caused by residual stress after a wafer thinning process. The oven reflow process using the ordinary reflow oven may not be suitable for fine pitch flip-chip bonding processes, because the process yield may be low.
Thermal compression bonding (TCB) process may be used for the flip-chip bonding process for the solder capped copper pillars with a small amount of solder. The TCB process is not a batch process unlike the oven reflow process. The TCB of the flip-chip IC is performed one chip at a time. The chip from the silicon wafer is flipped and transferred to a bond-arm with the bumps facing down. The chip is carried by a collet of the bond-arm, and the chip is dipped into a flux plate filled with a layer of flux for flux wetting of the bumps. The temperature of the bond-arm holding the chip should be at a low temperature, for example below 60° C., because most fluxes evaporate rapidly close to or above 80° C. The flux is used, because the flux helps to remove oxides on the surfaces of the bond pads and the solder caps, during bonding.
The chip is then aligned by an optical alignment module comprising an up-looking camera and a down-looking camera. The down-looking camera is used to align the substrate or the bottom chip. The chip is placed on a bonding location of the substrate or the bottom chip. A small compressive normal force is applied to the chip pressing it against the substrate or the bottom chip to ensure good contact between the solder capped copper pillars and the corresponding bond pads.
The bond-arm is heated up, to for example above 260° C., to bond the copper pillars to the bond pads of the substrate or the bottom chip. It is important to use an appropriate heater temperature profile for the bond-arm, to ensure solder joints are properly formed. Thereafter, the bond-arm cools down so that the solder solidifies and forms a joint, then the bond-arm releases the chip which is bonded to the substrate. The bond-arm rises to a stand-by position, and the bond-arm has to cool down to a temperature of less than 60° C. for the flux dipping of another chip for a next bonding cycle. Since cooling the bond-arm from above 260° C. to around 60° C. takes some time (for example more than 5 seconds), the overall cycle time for the TCB process will be time consuming. The flux dipping process is a bottleneck of the TCB process since the bond-arm has to cool down to a low temperature, for example below 60° C., to prevent rapid evaporation of the flux on the solder capped copper pillars.
It would be beneficial to increase the speed and accuracy of the bonding process as compared to the prior art.